:space_invader: Fix spacing of Clojure code

This commit:

  * Changes Loofah to use a HTML rather than an XML scrubber
  * Temporarily changes Gemfile to use a fork of Rouge that emits valid HTML
  * Makes some minor CSS changes to accomodate the newly valid HTML
  * Fixes #821

Once / if the pull request to Rouge is accepted the Gemfile should be
reverted to use the official version.
Merge pull request #838 from rsslldnphy/fix/clojure-whitespace

Fix spacing of Clojure code